In recent years, an optical signal transmission rate of an optical communication system that transmits an optical signal through an optical fiber has been rapidly accelerating so as to satisfy an increase in communication traffic as the Internet spreads. Likewise, a transmission rate of an optical transmitter-receiver has shifted from 2.5 Gb/s to 10 Gb/s, and research and development are underway with a view of realizing a transmission rate of 40 Gb/s. Accordingly, there an increasing needs to accelerate a transmission rate of a signal handled by an optical transmitter-receiver.
The optical transmitter-receiver converts a data signal to be transmitted from an electric signal to an optical signal and transmits the optical signal through a transmission optical fiber, on the other hand, receives an optical signal through a reception optical fiber and reproduces the optical signal received as an electric signal.
As an optical semiconductor package employed for the optical transmitter-receiver of this type, there are known can packages, box-like packages and the like. Conventional techniques using the can package are disclosed, for example, in Japanese Patent Application Laid-Open No. H6-314857 and Japanese Patent Application Laid-Open No. H11-233876.
Japanese Patent Application Laid-Open No. H6-314857 discloses a single-phase feed type optical semiconductor module that includes through lead pins sealed with glass. Japanese Patent Application Laid-Open No. H11-233876 discloses a technique for driving a laser diode by providing a pair of distant signal pins sealed with different dielectrics on a metal stem, connecting one of outputs of a differential driver to one of electrodes of the laser diode through one of the signal pins, and connecting the other output of the differential driver to the other electrode of the laser diode through a dummy load and a virtual grounding wire.
The single-phase can packages disclosed by these publications have the following disadvantages. Since they are single-phase can packages, impedance mismatching tends to occur in front and rear portions (portions in which the pins from the dielectric are exposed to an air layer) of a feed-through (a part in which the pins are covered with dielectric) when a modulated signal at 10 Gb/s or more is to be transmitted, and high frequency transmission characteristics of the package become worse. As a result, these single-phase can packages are being used for signal transmission only at up to about 2.5 Gb/s.
The technology disclosed in Japanese Patent Application Laid-Open No. H11-233876 is intended only to ensure stability during a high-rate operation by setting respective load impedances for the differential driver equal, but not to constitute the signal pins and a line from the signal pins to the laser diode into differential lines. In addition, a dummy resistor is disposed outside of the package, so that a signal quality degrades when a modulated signal is transmitted at 10 Gb/s or more. Further, according to this conventional art, a positive-phase differential signal and an antiphase differential signal are not applied to an anode and a cathode of the laser diode, respectively, so that the laser diode is not driven in a differential manner.
Conventional techniques using the box-like package are disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-164970 and Japanese Patent Application Laid-Open No. 2000-19473. Japanese Patent Application Laid-Open No. 2000-164970 includes a discloser about a single-phase feed type box-like package that connects a feed-through of a grounded coplanar substrate to a micro-strip substrate, and a single-phase feed type box-like package that connects a feed-through of a micro-strip substrate to the micro-strip substrate. Japanese Patent Application Laid-Open No. 2000-19473 includes a discloser about a single-phase feed type box-like package that connects a feed-through of a grounded coplanar substrate to a micro-strip substrate, a singe-phase feed type box-like package that connects a feed-through of a grounded coplanar substrate to the grounded coplanar substrate, and a single-phase feed type box-like package that connects a feed-through of a coaxial connector to a micro-strip substrate.
In such box-like packages, a micro-strip line is constituted by a ceramic substrate and a metallic pattern provided on an upper surface of the ceramic substrate, and a feed line can be formed with high accuracy so that an input signal supplied to a laser diode does not deteriorate much. Nevertheless, the box-like package has the following disadvantages. The ceramic substrate itself per unit area is expensive. If the feed-through is to be constituted, it is formed as a multilayer ceramic feed-through. In order to connect the multilayer ceramic feed-through to a lead, a brazing step or the like is necessary and it thereby takes time and labor so that the cost of the package rises. Further, because the ceramic package is used, the size of the package increases.
In the field of optical transmitters-receivers of this type, an optical semiconductor element module capable of realizing optical transmission at 10 Gb/s or more at low cost so as to spread optical communication not only to trunks but also to access systems for office and household purposes is strongly desired.
However, the conventional can package employed for the optical semiconductor element module as disclosed in Japanese Patent Application Laid-Open No. H6-314857 and Japanese Patent Application Laid-Open No. H11-233876 has the following disadvantages. Impedance mismatching tends to occur in front and rear portions of the feed-through and the high frequency characteristics of the package deteriorate. As a result, the can package cannot resist signal transmission at bit rates of 10 Gb/s or more.
In the box-like package employed in the conventional optical semiconductor element module provided with an external terminal made of ceramic as disclosed in Japanese Patent Application Laid-Open No. 2000-164970 and Japanese Patent Application Laid-Open No. 2000-19473, signal transmission at bit rates of 10 Gb/s or more can be realized; however, it has the following disadvantages. The ceramic substrate itself per unit area is expensive. If the feed-through is to be constituted, it is formed as a multilayer ceramic feed-through. In order to connect the multilayer ceramic feed-through to a lead, a brazing step or the like is necessary and it thereby takes time and labor so that the package becomes expensive.
It is an object of the present invention to provide a low cost optical semiconductor package capable of ensuring superior high frequency transmission characteristics, and performing a high-rate operation of 10 Gb/s or more.